Question

A car decelerates at a rate of 3.8 m/s2 for 12 seconds. The car’s initial speed was 65 m/s. What was the car’s final speed?

Answer 1

For an uniformly accelerated motion, the final velocity is given by:
$v_f = v_i +at$
where vi is the initial speed, a is the acceleration, and t the time interval.
In our problem, $v_i=65 m/s$, $a=-3.8 m/s^2$ (with negative sign, because the car is decelerating), and $t=12 s$. Substituting, 
$v_f = 65 m/s+(-3.8 m/s^2)(12 s)=19.4 m/s$